两次comboBox1绑定后报错..求解答

   private void AddNews_Load(object sender, EventArgs e)
        {
            try
            {
            label5.Text = Nowtime.ToString();
            string commandNew = "select * from News";
            AdapterNew = new SqlDataAdapter(commandNew, dbconn.connection);
            AdapterNew.Fill(dtNew,"News");
            dataGridViewNews.DataSource=dtNew.Tables["News"];
            string News_Class = "select * from News_Class";
            AdapterNew = new SqlDataAdapter(News_Class, dbconn.connection);
            AdapterNew.Fill(dtNew,"News_Class");
            comboBox1.DataSource = dtNew.Tables["News_Class"];
            comboBox1.DisplayMember = "ClassName";
            comboBox1.ValueMember="ClassID";
            }
            catch (Exception ex)
            {                MessageBox.Show(ex.ToString());
            }        }   private void comboBox1_SelectionChangeCommitted(object sender, EventArgs e)
        {
                comboBox2.Items.Clear();
                string News_SubClass = string.Format("select * from News_SubClass where ClassID={0}", comboBox1.SelectedValue.ToString());
                AdapterNew = new SqlDataAdapter(News_SubClass, dbconn.connection);
                AdapterNew.Fill(dtNew, "News_SubClass");
                //DataTable dt = dtNew.Tables["News_SubClass"];
                //DataRow dr = dt.NewRow();
                //dr["SubClassName"] = "测试";
                //dt.Rows.InsertAt(dr, 0);
                DataTable dt = dtNew.Tables["News_SubClass"]; ;
             //   comboBox2.DataSource = dtNew.Tables["News_SubClass"];
                //dtNew.Tables.Add(
                dtNew.Tables.Remove(dt);
                dtNew.Tables.Add(dt);
                comboBox2.DataSource = dtNew.Tables["News_SubClass"];
                comboBox2.DisplayMember = "SubClassName";
                comboBox2.ValueMember = "SubClassID";
        }
代码如上..求解